Switch mechanism for tracks

ABSTRACT

Switch mechanism associated with a vehicle supporting pair of main tracks and a pair of side tracks which has a pair of housings adapted to mount a pair of track portions at the juncture of the pair of main and side tracks. The track portions are mounted in their respective housings for generally horizontally disposed vertical movements between operative and inoperative positions. Cam means is mounted in the housings for movement thereof longitudinally of the track portions and means within the housing imparts simultaneous movements to the cam means in directions longitudinally of the track portions and to the track portions to move the track portions vertically into position to allow the vehicle to travel along the pair of main tracks or alternately switch the vehicle to the pair of side tracks.

United States Patent 1 1 3,591,799

(72] lnventor Harold L. Lice 1.399314 12/1921 Alonzo 246/433 2 l A I N2:33: wmom 55987 Primary Examiner-Arthur L. LaPoint I AssistantExaminer-Richard A. Bertsch [22] Flled Atzorney-Merchant and Gould 45Patented July 6, 1971 ABSTRACT: Switch mechanism associated with avehicle MECHAN'SM TRACKS iiiTgllftfi iliifiil"afi'ifiii $3321 i illf iii'iif'fifr" sclajms summing Flgs' tions at the juncture of the pair ofmain and side tracks. The [52] U.S.Cl 246/433 track portions are mountedin their respective housings for [51] Ill. Cl E01!) 7/08 generallyhorizontally disposed vertical movements between [50] Field 0' Search246/4 l 5, operative and inoperative positions, Cam means is mounted in313 the housings for movement thereof longitudinally of the trackportions and means within the housing imparts simultaneous [56]References Cned movements to the cam means in directions longitudinallyof UNITED STATES PATENTS the track portions and to the track portions tomove the track 855,156 5/1907 Barthel 246/433 portions vertically intoposition to allow the vehicle to travel 966,193 8/1910 Gardner 246/433along the pair of main tracks or alternately switch the vehicle1,194,441 8/1916 Teller 246/433 to the pair of side tracks.

PATENTEUJUL 61971 3459119 9 SHEET 1 [IF 3 INVENTOR. hiya/o L. bl'caPATENTEUJUL 6|97| 3,591 799 SHEET 3 OF 3 INVEN'H'NZ. Hero/o 4. 4/224 W 9M .55 6 ATTORNEY;

SWITCH MECHANISM FOR TRACKS BACKGROUND OF THE INVENTION ditional problemof snow and ice. Such snow or ice may enter the working mechanism in theform of either snow, ice or water. In the case of snow or ice themechanism-maybe jammed in much the same manner as it wouldbe in the caseof stones, rocks or other like foreign particles and in the'case ofwater such water may become frozen, thus rendering the movable partsimmovable. Also, prior art devices are extremely susceptible to rustand/or dust like particles which have the effect of causing the movableparts of the switching mechanism to become very hard to operate.

SUMMARY OF THE INVENTION It is a primary object of the present inventionto provide a switch mechanism for a pair of main tracks and apair ofside tracks extending therefrom which is substantially enclosed withinhousing means to protect same from the elements.

It is a further object of the present invention to provide a device ofthe character above described which isprovided with a liquid lubricantin order to maintain ease of operation thereof.

It is a still further object of the present invention to provide adevice of the character above described which is relatively inexpensiveto produce, durable in use, and foolproof in operation.

To the above ends there is provided a pair of housings, one each ofwhich is mounted on one of and between a pair of main tracks at thejuncture thereof with a pair of side tracks. Each of said housings ismounted below the uppermost surface of the tracks and serves to mount atrack portion for vertical movements between an operative position andan inoperative position. Cam means is mounted in each of the housings inunderlying relationship to opposite ends of each of the track portions,for movements longitudinally of the track portion and is engageable withthe opposite ends of a respective track portion whereby to support samein a generally horizontally disposed position during vertical movementsof the respective track portions. Means is mounted within each of thehousings for imparting simultaneous movements to the cam means indirections longitudinally of the respective track portions thereof andto the respective track portions in a vertical direction whereby to movethe track portions in a horizontally disposed, vertical direction toswitch a vehicle from the pair of main tracks to the pair of side tracksor permit a vehicle to maintain travel along the pair of main tracks.Each of the housings is filled with a liquid lubricant so as to enhanceease of operation thereof during switching movements of the trackportions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 2 is an enlarged view in verticalsection as seen generally from the line 2-2 of FIG. 1 portions thereofbeing broken away;

FIG. 3 is an enlarged fragmentary view in topplan of the switchmechanism of FIG. 1, portions thereof broken away and portions thereofremoved to better illustrate the working mechanism thereof;

, FIG. 4 is an enlarged vertical section as seen generally from the line4-4 of FIG. 1, portions thereof broken away;

FIG. 5 is a view in perspective of cammeans utilizedwith the presentinvention;

FIG. 6 is an enlarged vertical section asseen from the line 66 of FIG.1, portions thereof being broken away;

FIG. 7 is a fragmentary view in perspective of the manual i controlportion of FIG. 6; and

FIG. 8 is a fragmentary exploded perspective of a releasable couplingfor operatively connecting the manual control of FIG. 6 to automaticmeans for operating the switch mechanism.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly tothe drawings, and initially to FIG. 1 thereof, a pair of main linetracks 10, 11 are shown as being supported on a plurality of tieelements 12 extending transversely and spaced longitudinally of thetracks 10, 11. A

pair of side tracks l3, 14 extend from the main line tracks 10, 11 andare supported by the tie elements 12 at their point of juncture with themain tracks 10, 11.

Switch mechanism, indicated generally by the numeral 20, for switching avehicle'from travel along the main tracks 10, 11 onto the side tracks13,,14 includes housing means in the nature of a pair of housings 21,22. Housings 21, 22 are one each secured to one of the main line tracks10, 11 respectively, at the juncture of the tracks 13, 14 with thetracks 10, 11. Each of the housings 21, 22 are supported by an adjacentpair of tie elements 12 and their connection to theirrespective tracks10, 11. Each of the housings 21, 22 serves to mount substantiallyidentical mechanism for switching a vehicle from the main tracks 10, 11to the side tracks l3, l4, and, therefore, only the mechanism within thehousing 21 will be described in detail. Since the mechanism in thehousing 22 is substantially identical to that of housing 21, like partswill be identified in the drawings by like numerals with the lower caseletter a added.

As shown, particularly in FIGS. 2-4, housing 21; includes partitionelements 23, 24, which divides the housing 21- into first and secondhousing chambers 25, 26. Mounted inythe chamber 25, in juxtaposition tothe track 10, for vertical movements between a lowered inoperative andanelevatcd operative position, is a track portion 30. Track portion 30includes an upperportion which is tapered to define a camming surface 31that is engageable by the flange of a vehicle wheel,

shown by.dotted lines in FIG. 4, so as to switch same from track 10 totrack 13, and a lower portion which is of a dimension to be receivedwithin and guided by the wallsof housing '21 defining the first chamber25 during vertical movements thereof.

For the purpose of supporting opposite ends of the .track portion 30during vertical movements thereofland when track portion 30 is in.anelevated operative position, there .isprovided cam means in the natureof first and second incline ramps 35,36, see FIGS. '2, 3 and 5. Oppositeends of the track portion 30 are angularly relieved as at 37, 38, andeach of the inclined ramps 35, 36 is mounted, in underlying relationshipto the opposite ends of the track portion 30, with the upper 42 areformed on the upper surface of each of the terminal ends of the an'ns39, 40 and extend from the terminal end of their respective arms 39, 40for a distance toward a respective inclined ramp 35, 36. With theabove-described structure, the inclined ramp 35, 36 and their respectivearms 39, 40 are mounted in the housing 21 for movements longitudinallyof the track portion 30 during vertical movements of the track portion30 and serve to maintain the track portion 30 in a generally horizontalcondition during vertical movements thereof and when track portion 30 isin an elevated operative position.

Means mounted within the chamber 26 of housing 21 for impartingsimultaneous movements to the inclined ramps 35, 36 in directionslongitudinally of the track portion 30 and to the track portion 30 in avertical direction includes first and second gear trains indicatedgenerally by the numerals 45, 46.

Each of the gear trains 45, 46 includes a driving gear 47, 48respectively which is mounted in meshing engagement with each other andwith each one of the rack gears 41, 42 associated with a respectiveinclined ramp 35, 36. Driving gears 47 48 are each mounted for rotationon shafts 49, 50 mounted on a side wall of the housing 21 so as toposition the gears 47, 48 in overlying relationship to the rack gears41, 42. Shaft 50 is fixedly carried by the wall of housing 21 and drivegear 48 is rotatably mounted thereon. Drive gear 47 is fixedly mountedto the shaft 49 and shaft 49 is rotatably mounted in portion 30 betweenthe partition 23, 24 and terminates in the second chamber 26.Intermediate gears 55, 56 are rotatably journaled in the chamber 26 soas to have meshing engagement with the driving and driven gears 47, 48,51, 52 of a respective gear train 45, 46. It will be noted that all ofthe gears of the gear trains 45, 46 are mounted on horizontal axes whichare parallel and which extend transversely of the longitudinal-axis ofthe housing 21. With the above structure it will be seen that rotationof the shaft 49 and drive gear 47 carried thereby in a counterclockwisedirection will cause the driving gear 48 of gear train 46 to rotate in aclockwise direction and the respective rack gears 41, 42, arms 39, andinclined ramps 35, 36 to move in a longitudinal direction relative tothe housings 21 wherein the upper surface, of the inclined ramps 35, 36engage an adjacent surface of a respec tive portion 37, 38 of the trackportion 30. Simultaneously, the intermediate gears 55, 56 and drivengears 51, 52 associated therewith are driven in a direction to cause therack gears 53, 54 and track portion 30 associated therewith to be movedin a vertically upward direction to a position wherein the uppermostsurface of the track portion 30 will lie generally in the same plane asthe upper surface of track 10. As seen particularly in FIGS. 3, 4 and 5,rack gears 41, 42 are laterally offset from one another transversely ofchamber 26 so as to overlap when the arms 39, 40 and inclined ramps 35,36 are moved to the above-described position. Also, by reference to FIG.2, it will be noted that rack gear 41 and the axis of drive gear 47 areslightly elevated with respect to rack gear 42 and the axis of drivegear 48. In this manner, during the abovedescribed movements of the rackgears 41, 42, rack gear 42 will pass under drive gear 47 withoutengaging same. Further, it will be noted by reference to FIG. 3, thatdrive gear 48 has a dimension along its axis of rotation substantiallyless than that of drive gear 47. This provides proper clearance for thepassage of rack gear 41 without engagement with drive gear 48 during theabove-described movements.

For the purpose of enclosing the housing 21, a cover element 57 ismounted to close the upper side of the housing 21. A plurality of bolts58 have threaded engagement with the side walls of the housing 21 andpartition elements 23, 24, and hold the cover element 57 and partitionelements 23, 24 securely in place. Cover element 57 has an opening 59formed therein which corresponds generally to the shape of the upperportion of the track portion 30 and allows the upper portion thereof tomove from the full line position of FIG. 4 to an elevated operativeposition for switching a vehicle. In order that the internal workingparts of housing 21 are maintained in a generally rust free condition,the housing 21 is filled with a liquid lubricant 60. As shown in FIG. 4,when the track portion 30 is in its lowered inoperative position, theliquid lubricant 60 substantially fills the interior of the housing 21.As such, the liquid lubricant 60 not only lubricates the internal partsof the housing 21, thus assuring ease of operation thereof withoutbinding, but in cooperation with cover 57, tends to discourage theentrance of foreign material, such as water and/or granular particles,into the housing 21 which may subsequently cause malfunction of theswitching mechanism 20.

It will be noted by reference to FIG. 3 that the terminal end ofsidetrack 13 is formed to overlie and rest upon an angularly relievedportion 61 formed on an adjacent end of housing 21. Also, track 11 isformed to overlie and rest on an angularly relieved portion 61a formedon opposite ends of the housing 22. Thus, adequate support is providedfor the tracks 13, 11 in the vicinity of the housings 21, 22 ofswitching mechanism 20.

Means for simultaneously moving a track portion 30, 30aassociated withone of the housings 21, 22 to a vertically elevated operative positionand a track portion 30, 30aassociated with the other of the housings 21,22 to a vertically lowered inoperative position includes differentialgear means, indicated generally by the numeral 64, and control meanspositioned laterally of the pair of main tracks, indicated generally bythe numeral 65. As seen particularly in FIGS. 1 and 4, differential gearmeans 64 includes a housing 66 mounted on the tie elements 12 whichsupport the housings 21, 22. The housing 66 has a pair of shafts 67, 68journaled for rotation therein on coaxial axes. A pair of bevel gears69, 70 are fixedly carried by adjacent ends of the shafts 67, 68,respectively, within housing 66, for common rotation with theirrespective shafts 67,68. An intermediate bevel gear 71 is rotatablymounted on the housing 66 and has meshing engagement with each of thebevel gears 69, 70. The outer end of each of the shafts 67 68 arerespectively coupled to the shafts 49, 49aassociated with housings 21,22 and drive gears 47, 47a respectively mounted therein. I v

7 It will be noted that the switching mechanism associated with housing22 differs from the switching mechanism associated with housing 21 inthat the upper portion of track portion 30a is generally rectangular inhorizontal cross section and that cam surface Slathereof extends in aline generally parallel to the longitudinal direction of track 11. Itwill be also noted that track 11 at the juncture thereof with track 14is relieved transversely thereof in an amount equal to the transversedimension of the upper portion of track portion 30afor a reason whichwill become apparent hereinafter during a description of the operationof the switching mechanism 20.

Mounted for rotation in the housing 66 of differential gear means 64 andextending laterally therefrom along an axis parallel to the shafts 67and 49 is a rotary shaft 75. Fixedly carried by the shaft 75, within thehousing 66, is a bevel gear 76. Bevel gear 76 has meshing engagementwith the bevel gear 79 and upon rotation of the gear 76 rotation isimparted to the gears 69, 70 and 71. Shaft 75 extends laterallyoutwardly from the housing 66 in underlying relationship to the housing21 and track 10 and has its outer free end journaled for rotation in ahousing or standard 77 of switch control means 65. Housing or standard77 is fixedly mounted on adjacent ones of the tie elements 12 whichunderlie the housings 21, 22 and mount the differential gear means 64. Abevel gear 78 is fixedly carried by the extended end of the rotary shaft75 and has meshing engagement with a bevel gear 79 carried by one end ofa generally vertically extended rotary shaft 80 which is mounted forrotation in the standard 77. An electrically operated reversible gearhead motor 81, connected to a source of power not shown, is mounted inthe housing or standard 77 and includes a power output shaft having abeveled gear 82 fixedly mounted thereon. Bevel gear 82 has meshingengagement with a bevel gear 83 fixedly carried by the vertical shaft80, and upon energization of the gear head motor 81, rotation of thegear 82 imparts rotation to the gear 83 and shaft 80.

Means for manually rotating the shaft 80 includes a vertical extension84 of shaft 80 which is releasably coupled to shaft 80 as at 85. Ahandle 86 extends laterally from the shaft 84 through a slot 87 formedin the side wall of the housing or standard 77 and terminatesexteriorally of the housing 77. Generally vertically and laterallyextended notches 88, 89 are formed at opposite ends of the slot 87 andmovement of the handle 86 upwardly and laterally into one of the notches88, 89 disengages the shaft 84 from the shaft 80.

DESCRIPTION OF THE OPERATION As shown particularly in FIGS. 1-4, thetrack portions 30, 30a are positioned to maintain the travel of thewheels of a vehicle along the main line tracks l0, 11. That is, trackportion 30a is shown in an elevated operative position and track portion30 is shown in a lowered inoperative position. In this position theflange of the wheel supported on track 11 is prevented from entering agroove 90 formed at the juncture of the tracks ll and 14 by cam surface31a. At the same time, a wheel supported on the track is allowed to-passover the upper portion of the track portion 30 out of engagement withcam surface 31 thereof and enter a groove 91 formed between the tracksl0, 13 at the juncture thereof, thus allowing the wheels of the vehicleto travel along the main tracks 10, l I.

When it is desired to switch the wheels of the vehicle from travel alongthe main tracks 10, 11 onto the side tracks l3, 14, the handle 86 isoperated to move same from left to right with respect to F IG. 7 oralternatively the motor 81 is energized to rotate the shaft 80 in acounterclockwise direction. This causes movement of the shaft 75 in acounterclockwise direction, shafts 67, 49 in a clockwise direction, andshafts 68, 49ain a counterclockwise direction. Movement of the shaft 49in a clockwise direction imparts the correct rotation tothe gear trains45, 46 to simultaneously raise the track portion 30 and move theinclined ramps 35, 36, longitudinally of the housing 21, toward eachother until the uppermost surface of the track portion 30 is coplanarwith the uppermost surface of the track 10. At the same time, movementof the shaft 49a in a counterclockwise direction imparts the correctrotation to the gear trains 45a, 46ato cause movement of the track'portion 30a toward a vertically lowered inoperative position andinclined ramps 35a, 36a, longitudinally of the housing 22, away from oneanother to a position such as that illustrated in FIG. 2 relative toinclined ramps 35, 36.

When the track portions 30, 30a have assumed their respective verticallyelevated operative position and lowered inoperative position abovedescribed, the flange of the wheel supportedon track 10 engages the camsurface 31 to guide such wheel onto track 13. At the same time the wheelsupported by track 11 moves into overlying relationship to the trackportion 30aand onto track 14. Since the track portion 30a has been movedto a lower inoperative position, the flange of the wheel moving onto thetrack 14 is allowed to pass thereover and into the groove 90 between thetracks 11, 14.

Due to the construction of supporting structure for wheels normallyassociated with track guided vehicles, wherein the wheels are maintainedin a substantially fixed distance transversely of the tracks, thecamming action of the surface 31, associated with track portion 30,imparts a like camming action to the wheel supported on track 11. Thus,the wheel on track 1 1 is caused to enter the slot 90 and move ontotrack 14, while the wheel supported by track 10 moves onto track 13.Reversing of the above-outlined procedure obviously returns the trackportions 30, 30a to their full line position shown in H6. 4 andsubsequent vehicles traveling along the tracks 10,

11 are allowed to bypass the side tracks 13, 14 due to cammi ng actionof surface 31 a on the wheel supported on track 11.

it can be seen from the above description that the moving parts of theswitch mechanism 20 are mounted within their respective housings 21, 22in a manner to permit free vertical movements between verticallyelevated operative positions and vertically lowered inoperativepositions without binding of the moving parts and with a minimumof'friction. Also, by substantially enclosing the moving partswithin-the housings.

21, 22 and within a liquid lubricant, there is less likelihood thatforeign particles, such as sand, snow, and ice, or water, which may turnto ice during the colder winter months, may enter the housings 21, 22causing malfunction of the switch mechanism 20. Thus, a switchingmechanism for tracks has been. provided which results in safe,dependable and maintenance-free operation over prolonged periods oftime.

lclaim:

1. Switch mechanism for use with a pair of vehicle supporting maintracks and a pair of side tracks extending from said main tracks andcomprising:

a; elongated housing means adapted to. be mounted adjacentthe point ofjuncture of thepair of main and side tracks in juxtaposition to one ofthe tracks between the pair of main tracks and below theuppermostsurface of the tracks, said housing means including a pair ofhousings one each of which is mounted on one of and between the pair ofmain tracks at the .juncture thereof with the pair of sidetracks;

b. a track portion formed of a portion of one of the main and sidetracks mounted in each of said housings for vertical movements betweenan operative position and an inoperative position and includinglongitudinally spaced.

vertically extended rack gear portions; c. cam means mounted in each ofsaid housings for movements longitudinally of said track portion andengageable.

with opposite ends thereof whereby to support said track portion in ahorizontally disposed position when said track portion is in anoperative position; and

d. means mounted within each of said housings forimparting simultaneousmovements to said cam means in directions longitudinally of said trackportion and to said track portion in said vertical direction whereby tomove said track portion in a horizontally disposedv vertical directionto a position to switch a vehicle from the pair of main tracks to thepair of side tracks, said means for im-. parting simultaneous movementsto each of said cam means and their respective track portions comprisinggear means including first and second gear trains mounted in each ofsaid housings in meshing engagement with each other and one of said rackportionsof a respective track portion to move said track portionsvertically to said operative and inoperative positions and said cammeans longitudinally of said track portions.

2. The structure of claim 1 in which end of said cam means comprises:

a. first and second inclined ramps mounted in each of said housings inunderlying relationship to opposite ends of respectivev track portions;

b. an arm extending laterally from each of said inclined ramps andlongitudinally of respective ones of said track portions;

c. a rack gear formed on the extended end arms; and

d. one each of said rack gears having meshing engagement with oneof saidgear trains whereby rotation of said gear trains to.move saidrespectivetrack portions in vertical directionscauses respectiveinclined ramps to move in a direction longitudinally of said respectivetrackportions to maintain same in a generally horizontal condition.

3. The structure of claim 2 in whicheach of'said gear trains includes:

of each of said a. a driving gear mounted in meshing engagement witheach other and one of said rack gears associated with one of saidinclined ramps;

b. a driven gear mounted in meshing engagement with one of said rackgears of said track portions; and

c. an intermediate gear mounted to have meshing engage ment with itsrespective driving and driven gear.

4. The structure of claim 1 in which each of said housings is filledwith a liquid lubricant.

5. The structure of claim 3 in further combination with meansoperatively connected to one of the pair of driving gears associatedwith one of the housings and one of the driving gears associated withthe other of said housings for simultaneously moving a track portionassociated with said one housing to a vertically elevated operativeposition and a track portion associated with said other of said housingsto a vertically lowered inoperative position.

6. The structure of claim 5 in which said means includes:

a. a drive shaft extending from a given one of said drive gears of eachof said housings;

b. differential gear means connected to the free end of each of saiddrive shafts and adapted to simultaneously drive same in oppositedirections of rotation; and

c. control means positioned laterally of the pair of main tracks andoperatively connected to said differential gear means to impart rotationthereto.

7. The structure of claim 6 in which said last mentioned means includes:

a. a rotary shaft operatively connected to said differential gear means;

b. an electrically operated reversible gear head motor;

c. means including a standard operatively connecting said motor to saidrotary shaft to drive same in a selected direction of rotation; and

d. means releasably coupled to said rotary shaft for manually rotatingsame.

8. The structure of claim 2 in which said rack gear associated with oneof said inclined ramps in each housing is laterally offset with respectto said rack gear associated with the other of said inclined ramps ineach housing and in which said one of said rack gears in each of saidhousings is vertically offset with respect to the other of said rackgears in each of said housings.

1. Switch mechanism for use with a pair of vehicle supporting maintracks and a pair of side tracks extending from said main tracks andcomprising: a. elongated housing means adapted to be mounted adjacentthe point of juncture of the pair of main and side tracks injuxtaposition to one of the tracks between the pair of main tracks andbelow the uppermost surface of the tracks, said housing means includinga pair of housings one each of which is mounted on one of and betweenthe pair of main tracks at the juncture thereof with the pair of sidetracks; b. a track portion formed of a portion of one of the main andside tracks mounted in each of said housings for vertical movementsbetween an operative position and an inoperative position and includinglongitudinally spaced vertically extended rack gear portions; c. cammeans mounted in each of said housings for movements longitudinally ofsaid track portion and engageable with opPosite ends thereof whereby tosupport said track portion in a horizontally disposed position when saidtrack portion is in an operative position; and d. means mounted withineach of said housings for imparting simultaneous movements to said cammeans in directions longitudinally of said track portion and to saidtrack portion in said vertical direction whereby to move said trackportion in a horizontally disposed vertical direction to a position toswitch a vehicle from the pair of main tracks to the pair of sidetracks, said means for imparting simultaneous movements to each of saidcam means and their respective track portions comprising gear meansincluding first and second gear trains mounted in each of said housingsin meshing engagement with each other and one of said rack portions of arespective track portion to move said track portions vertically to saidoperative and inoperative positions and said cam means longitudinally ofsaid track portions.
 2. The structure of claim 1 in which end of saidcam means comprises: a. first and second inclined ramps mounted in eachof said housings in underlying relationship to opposite ends ofrespective track portions; b. an arm extending laterally from each ofsaid inclined ramps and longitudinally of respective ones of said trackportions; c. a rack gear formed on the extended end of each of saidarms; and d. one each of said rack gears having meshing engagement withone of said gear trains whereby rotation of said gear trains to movesaid respective track portions in vertical directions causes respectiveinclined ramps to move in a direction longitudinally of said respectivetrack portions to maintain same in a generally horizontal condition. 3.The structure of claim 2 in which each of said gear trains includes: a.a driving gear mounted in meshing engagement with each other and one ofsaid rack gears associated with one of said inclined ramps; b. a drivengear mounted in meshing engagement with one of said rack gears of saidtrack portions; and c. an intermediate gear mounted to have meshingengagement with its respective driving and driven gear.
 4. The structureof claim 1 in which each of said housings is filled with a liquidlubricant.
 5. The structure of claim 3 in further combination with meansoperatively connected to one of the pair of driving gears associatedwith one of the housings and one of the driving gears associated withthe other of said housings for simultaneously moving a track portionassociated with said one housing to a vertically elevated operativeposition and a track portion associated with said other of said housingsto a vertically lowered inoperative position.
 6. The structure of claim5 in which said means includes: a. a drive shaft extending from a givenone of said drive gears of each of said housings; b. differential gearmeans connected to the free end of each of said drive shafts and adaptedto simultaneously drive same in opposite directions of rotation; and c.control means positioned laterally of the pair of main tracks andoperatively connected to said differential gear means to impart rotationthereto.
 7. The structure of claim 6 in which said last mentioned meansincludes: a. a rotary shaft operatively connected to said differentialgear means; b. an electrically operated reversible gear head motor; c.means including a standard operatively connecting said motor to saidrotary shaft to drive same in a selected direction of rotation; and d.means releasably coupled to said rotary shaft for manually rotatingsame.
 8. The structure of claim 2 in which said rack gear associatedwith one of said inclined ramps in each housing is laterally offset withrespect to said rack gear associated with the other of said inclinedramps in each housing and in which said one of said rack gears in eachof said housings is vertically offset with respect to the other of saidrack gears in each of said housiNgs.